Demo meters are essentially large scale versions of the ammeters or voltmeters you may have in school. The difference with these is that you can easily swap the fascia plates to customise the meter to your chosen application. There are many plates commercially available which fit these meters meaning that these devices should last many years. You may still have wooden versions from the ‘70s in your cupboards which should still accommodate these fascias.
These meters are designed to be used for demonstrations where students may have trouble reading a smaller scale. They are fairly expensive for what they are but will last a long time, especially if only used by staff!
The 0-5 and 0-10A plates along with the 0-5 and 0-10V plates should suit most school applications. Milliamp and millivolt fascias may still be available. If you have older versions, you may even have fascias which measure temperature.